I've an old QR one and it's stiff enough. I did manage to rip the QR out over the lawyer tabs (crash) landing a jump, but that's a fault of QR's in general, not the fork. And it was a few miles later before I noticed the front wheel was loose.

Not on the Salsa website as it's been superseded by a 15mm fork (which isn't on the Salsa website yet either, but is on Ison's). AC on the 20mm fork is less than the 15mm (presumably because the former is 80mm suspension corrected, and the latter 100mm?)

IIRC there was something about needing internal anti-corrosion treatment as part of the Maxle licence which is why, together with the cost of the axle, it's double the price.

This is my concern. As much as I love the idea, can a 20mm maxle really make it that much stiffer?

The argument for bolted rigids is mainly because your sus fork is bolted too and you want to swap over to rigid for winter using the same wheels. Any (minor I'd expect) stiffness difference isn't likely to make any difference unless somehow you're already pushing a rigid that hard over the rough, ie have wrists of steel.

I emailed Salsa to ask the same questions as those in my op and got an impressively quick reply. Apparently the 20mm Maxle is stiffer than QR, and it's not on the website as they are phasing them out in favour of 15mm ones (corrected for 100mm travel). So pretty much as per @rathertobeintobago's post then.

So I believed the hype and spunked the money. Sorry jameso, you seem like a good man. I was in a lose-lose situation: either hate myself for spending the money or put up with a niggling feeling that the forks could be better every time I rode them. The fact that I chose the former tells you all you need to know about me as a person.

Security and stiffness are nice, whether benefits are felt or imagined.

But compatibility would be the seller for me. I'd have spent a fair bit extra on a 15mm Maxle 425mm Cromoto if it was available. The wheels are shared between the bike with the Revs and the bike with the Cromotos, and the hub adapters can't be swapped without taking the disc rotor off first.